This invention is directed to a method for the preparation of finely divided, electrically conductive tin(IV) oxide, wherein an aqueous solution of tin(IV) oxalate and/or the oxalates of polymeric tin(IV) compounds is prepared in a first step, and the desired tin(IV) oxide is obtained from this aqueous solution in a second step. The invention is directed also to the use of the tin(IV) oxide obtained by the inventive method, as a filler or pigment in plastics, lacquers, paints, papers, textiles and toners.
The application of electrically conductive and infrared-reflecting tin oxide layers on substrates, particularly on glass, has long been known. Frequently, such electrically conductive and infrared-reflecting tin oxide layers on glass are prepared by the pyrolytic decomposition of liquid preparations, which contain, essentially, one or more tin parent compounds and one or more suitable fluoride-containing doping compounds.
The electrical conductivity is produced by defects in the respective tin oxide layers, such defects being largely formed by the doping material that has been added. Defect terms or donor terms are produced which are only a little below the conduction band and from which, if necessary, electrons can be brought into the conduction band with little expenditure of energy. This is not the case with pure, undoped tin oxide because of the forbidden zone that exists between the valency band and the conduction band.
Electrically conductive pigments are required to satisfy the need for electrical conductivity or antistatic adjustment of various solid and liquid industrial products, such as plastics, paints and lacquers, papers, toners and textiles. Aside from metal powders and graphite, which necessarily cause a dark coloration in such industrial products, powdery semiconductors are frequently used. For this purpose, it is desirable to employ finely grained semiconductor pigments of high electrical conductivity or low specific resistance which are as lightly colored as possible or even white.
In EP-A-O 235 968, a transparent aqueous solution is described which contains a tin compound and which is obtained by the reaction of tin carboxylate, particularly tin oxalate, with hydrogen peroxide in the ratio of 1:1.5 or above in an aqueous medium. At the same time, a doping agent can be present in the reaction system in an amount of 0.01 to 0.35 moles per mole of tin carboxylate. This transparent, aqueous solution can then be calcined at a temperature above 400.degree. C.
However, this method is not satisfactory in every respect. Tin(II) oxalate, which is preferably used as starting material, must first of all be synthesized from metallic tin or a tin(II) compound. Since tin(II) oxalate is rather insoluble in water, the oxidation with hydrogen peroxide proceeds in an aqueous medium in a heterogeneous phase. Moreover, tin(IV) oxide obtained by calcining at temperatures of more than 500.degree. C. is crystalline to a considerable extent, whereas for application reasons, amorphous oxide powder is desired.